Powder production by gas atomization of liquid metal

ABSTRACT

This method comprises striking a stream of liquid metal with a jet of gas which is rotated about the liquid metal stream so that it strikes the latter at an angle of between about 5* and about 90* with respect to the flow axis of the liquid stream to break up the liquid metal into small drops which solidify as a powder. The width of the gas jet is preferably less than that of the liquid metal stream. The rotational velocity of the gas jet about the liquid metal stream is sufficient to circle the liquid stream at least once before the latter moves out of the gas impingement zone. The apparatus employed comprises a vertically-aligned tube which is rotatably mounted in a stationary housing which, together, define an annular plenum chamber located concentrically about the tube. The wall of the rotatable tube defines an aperture which is angled between about 5* and about 90* with respect to the axis of the tube so that a stream of gas flowing therethrough is centered on the vertical axis of the rotatable tube along which liquid metal is gravity flowed from a pour cup positioned above the rotatable tube. As the latter is rotated, a gas is flowed into the plenum chamber and through the angled tube aperture to strike the liquid metal stream flowing downwardly through the tube. The resulting fine metal particles are collected after solidification in a chamber placed below the rotatable tube.

United States Patent [191 Kaufmann [54] POWDER PRODUCTION BY GAS ATOMIZATION OF LIQUID METAL Albert R. Kautmann, Lexington,

Mass.

[73] Assignee: Whittaker Corporation [22] Filed: Nov. 26, 1971 21] App]. No.: 202,351

[75] Inventor:

6/1971 Hegmann ..264/l2 Primary Examiner-Robert F. White Assistant Examiner-J. R. Hall Attorney-Donald E. Nist et al.

[57] ABSTRACT This method comprises striking a stream of liquid metal with a jet of gas which is rotated about the 1 Apr. 3, 1973 liquid metal stream so that it strikes the latter at an angle of between about 5 and about 90 with respect to the flow axis of the liquid stream to break up the liquid metal into small drops which solidify as a powder. The width of the gas jet is preferably less than that of the liquid metal stream. The rotational velocity of the gas jet about the liquid metal stream is sufficient to circle the liquid stream at least once before the latter moves out of the gas impingement zone.

The apparatus employed comprises a verticallyaligned tube which is rotatably mounted in a stationary housing which, together, define an annular plenum chamber located concentrically about the tube. The wall of the rotatable tube defines an aperture which is angled between about 5 and about 90 with respect to the axis of the tube so that a stream of gas flowing therethrough is centered on the vertical axis of the rotatable tube along which liquid metal is gravity flowed from a pour cup positioned above the rotatable tube. As the latter is rotated, a gas is flowed into the plenum chamber and through the angled tube aperture to strike the liquid metal stream flowing downwardly through the tube. The resulting fine metal particles are collected after solidification in a chamber placed below the rotatable tube.

7 Claims, 2 Drawing Figures 1 POWDER PRODUCTION BY GAS ATOMIZATION OF LIQUID METAL BACKGROUND OF THE INVENTION tion to break up liquids into drops is well known. Powders can be produced using this method by hitting a liquid metal stream with a jet of high velocity gas to cause liquid drops to form. solidification of these drops produces the desired powder.

Gas atomization techniques take advantage of the transfer of kinetic energy from the gas stream to the liquid to create the surface energy required for drop or particle formation. Presently-employed gas atomization techniques attempt to surround or enclose a liquid stream with high velocity gas so that the liquid cannot readily escape from the region of high velocity. This is to ensure a relatively long contact time between the liquid and gas streams since the magnitude of the transfer of energy from the gas to the liquid is directly proportional to the contact time.

Although a variety of orifice shapes and configurations have heretofore been employed to produce the jet of gas, the presently-available techniques generally employ one or more stationary orifices (usually two orifices located on opposite sides of the liquid stream) through which a gas is flowed to strike the liquid metal stream. The gas streams are wider than the liquid metal streams to prevent the liquid metal from escaping sideways from the gas stream.

These techniques have several disadvantages including the use of disproportionately high volumes of gas per unit weight of metal powder product and a generally low yield of very fine powder. Apparently these disadvantages result both from the contact of onlyv a small portion of the gas with the liquid metal because of the need to use gas streams of greater width than the liquid stream and from the re-agglomeration of liquid particles during the gas-liquid contact period probably because of the turbulence caused by these techniques. Estimates indicate that only about 1 percent of the gas kinetic energy is presently employed processes is transferred to the liquid and that, of this transferred amount of energy, only about One-half is used to create the required surface energy for the particles.

SUMMARY OF THE INVENTION The method of this invention comprises rotating a gas jet about a liquid metal stream so that the gas jet strikes the liquid stream at an angle between about 5 and about 90 to the flow axis of the liquid stream and so that it circles the liquid stream at least once before the liquid moves out of the gas impingement zone. The width of the gas jet is preferably less than that of the liquid stream. The particles are collected as powder after they are allowed to solidify before contacting any surfaces.

One of the advantages of this method is the low volume of gas required to produce a unit of metal powder. Another advantage is that extremely fine powder can be produced in substantial quantity. A further advantage is that the fineness of the powder produced can be varied by changing the rate at which the gas jet revolves about the liquid stream. Additionally, the foregoing advantages are obtained using relatively simple apparatus.

DESCRIPTION OF THE DRAWING FIG. 1 is an elevational cross-sectional view of apparatus used to perform the method of this invention.

FIG. 2 is a partial sectional view of the apparatus of FIG. 1 taken along the line 2-2 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT In general, the herein-described method comprises striking a stream of liquid metal with a gas jet by rotating the gas jet about the liquid stream so that the jet encircles the liquid stream at least once before the liquid metal leaves the impingement zone. The gas jet is angled with respect to the flow direction or axis of the liquid metal stream so that it strikes the latter at an acute angle, with the gas moving in the same general direction as the liquid metal. The gas jet is positioned with respect to the liquid metal stream so that the flow axis of the gas jet lies in the planes extending radially through the flow axis of the liquid metal stream. The width of the gas jet is made less than that of the liquid metal stream.

While it is not presently known for certain why substantially improved results are obtained using the herein-described method, it is believed that, by using a gas jet narrower than the liquid metal stream, there is a tendency for the gas jet to peel off the surface of the stream in a progressive manner as compared with prior techniques which attempted to accelerate the whole liquid stream cross-section at once. It is believed that this tendency results in a better change of applying forces over microscopic dimensions in the liquid metal. Since the application of tensile and shear forces to the liquid on a microscopic scale comparable to the diameter of the particles to be formed is believed to be the major factor for the formation of fine'particles, rather than merely having sufficient energy available on a macroscopic level to form the particles, it is believed that the aforementioned tendency of this method is responsible for the fine powders produced by this.

powder and the degree of fineness of the powder, in-

" creases as the rotational rate of the gas jet increases.

This is believed to be due to the chopping effect of the gas stream on the liquid stream as the former circles the latter. I j

The angle at which the gas jet strikes the liquid metal stream varies between about 5. and about with respect to the flow axis of the liquid stream. At angles less than about 5, the production of powder substantially decreases, possibly because of insufficient penetration of the liquid stream by the gas. At angles approaching 90, powder production also substantially decreases apparently, in this case, because of the escape of substantial amounts of the liquid metal from the gas jet and the recombination of some of the particles due to the excessive turbulence at impingement angles approaching 90. Preferably, the impingement angle employed is less than about 45 to significantly eliminate the escape of particles from the gas jet impingement zone to thereby optimize both the qualitative and quantitative yield of powder.

The cross-sectional width or diameter of the gas jet is preferably less than that of the liquid metal stream to maximize the efficiency of this process by causing all of the gas to contact the liquid metal. As previously described, prior art processes reversed this width relation to prevent liquid metal from escaping from the gas impingement zone so that some of the gas was employed for this purpose rather than for providing the shear forces necessary to produce powder. By contrast, the herein-described method maintains the particles in the gas impingement zone by the striking of the liquid metal by the gas jet from all sides in a continuous chopping manner. The ratio of the width of the gas jet to that of the liquid metal stream is less than one but is variable below that value. Satisfactory results have been obtained using a ratio of about 0.5.

The gas employed is usually a gas which is inert to the metal being powdered to prevent oxidation or other contamination of the metal. For example, argon, helium and nitrogen have proved useful. However, with metal systems where oxidation is not a concern, other fluids for atomization such as air, water or steam may be utilized.

' The gas pressure employed is at least sufficient to penetrate the liquid metal stream. Gas pressures on the order of 200 psi-500 psi have been used satisfactorily although both lower and higher pressures could be employed. It has been found that gas pressures on the higher end of this range produce better results than the gas pressures on the lower end of this range.

' This invention will now be further described with respect to the Figures in which the numeral designates the powder-forming apparatus of this invention. A liquid metal stream 12 is formed by allowing cup 16, to exit therefrom through an opening 18 in the bottom wall 20 thereof. The pour cup 16 is positioned above a cylindrical tube or rotator 22 having a bore 23 so that the flow axis of the liquid metal stream is colinear with the longitudinal axis of the rotator 22. However, it will be understood that the same relation between liquid metal stream and rotator may also be obtained by pouring the liquid stream from a tundish appropriately positioned to yield colinearity with the rotator.

The rotator 22 has a flange collar 24 intermediate its ends which is supported on an annular bearing 25 which is retained in a recess 26 in a housing 28. The upper end of the rotator 22 (as seen in FIG. 1) is provided with a pulley 30 which is driven through a belt 32 by a motor (not shown).

The rotator 22 extends through a cylindrical bore 34 in the housing 28 which is sized to slidably receive the rotator. The housing 28 defines an annular channel 36 which is axially aligned with the rotator 22 and which surrounds the lower end section of the rotator. The

walls of the housing 28 which define the channel 36 together with the wall of the rotator 22 define an annular plenum chamber 38. The latter is in communication, at its periphery, with a gas pressure source (not shown) through an inlet duct 40 in the housing 28. The plenum chamber 38 is also in communication with the bore 23 of the rotator 22 through an aperture 42 extending through the rotator wall. The aperture 42 is oriented with respect to the axis of the'rotator (as shown in FIG. 2) so that the axis of the aperture lies (as the rotator rotates) in the vertical planes extending radially trough the rotator axis. Additionally, the aperture 42 is slanted through the rotator wall 22 so that the gas jet exiting therefrom into the rotator bore 23 strikes the liquid metal stream 12 at the desired angle between about 5 and about to the flow axis of the liquid stream.

The aperture 42 is preferably a slit but may be a cylindrical aperture. Additionally, the aperture 42 may comprise two or more spaced, axially-parallel, cylindrical apertures as shown in FIG. 1. As the width of the aperture 42 increases, in the rotator length direction, the width of the gas jet increases thereby increasing the height or length of the impingement zone 44. The latter increase, of course, results in a longer time during which any portion of the liquid metal stream 12 is struck by the gas jet (assuming a constant velocityfor the liquid metal in the impingement zone 44).

Preferably, annular seals 46 are provided between the facing surfaces of the housing 28and rotator 22 walls both above and below the plenum chamber 38. This is to ensure that substantially all of the gas entering the plenum chamber 38 exits therefrom through the rotator aperture 42.

In operation, the height of the pour cup 16 above th impingement zone 44 is first adjusted to provide the liquid metal 14 with a desired velocity as it passes through the impingement zone 44. A rotator 22 having an aperture 42 oriented at the desired angle with respect to the longitudinal axis of the rotator and sized to provide an impingement zone of desired length is positioned within the housing bore 34 and connected to a drive motor. The pour cup 16 is filled with metal 14 which is preferably melted therein. Gas at the desired pressure is flowed through inlet duct 40 in the housing 28 and from there into the plenum chamber 38 from which it exits as a jet 47 through the rotator aperture 42. f

The rotator 22 is caused to rotate at thedesired rpm so that the gas jet 47 will circle the liquid metal stream 12 at least once during its passage through the impingement zone 44. Thereafter, the liquid metal 14 is allowed to flow through the hole 18 in the bottom 20 of the pour cup 16 so that it is axially aligned with the rotator 22. I

As soon as the liquid metal 14 enters the impingement zone 44, it is struck by the gas jet 47. The resulting shear forces on the liquid metal stream 12 cause it to break up into fine particles or powder 48. These particles 48 are allowed to cool andsolidify by passage through an atmosphere, which may be an inert gas, before they are collected in a container (not shown).

This invention will now be further described by the following Examples.

EXAMPLE 1 This Example illustrates the improvement obtained The apparatus employed was substantially as shown in the Figures except that, in place of the gas seals 46, air gaps of about 1 mil were left between the rotator and housing. This allowed some of the gas entering the housing to escape therefrom without passing through the aperture in the rotator. The tube employed as the rotator was a 1.5 in. ID. graphite tube with a 0.25 in. wall. The rotator aperture consisted of two closelyspaced holes (as shown in FIG. 1) drilled through the wall of the rotator at an angle of 20 to the flow axis of the liquid metal stream. An induction heated pour cup was positioned above the rotator so that the liquid metal fell about six inches before encountering the gas jet. The metal employed as tin which was heated to a temperature of about 500F in the pour cup before being allowed to flow through a 0.187 in. diameter hole in the bottom of the pour cup. Argon gas at 200 psi was employed to produce the gas jet.

The rotator velocity was set at 2500, 4300, 8200 and 9800 rpm. The solid metal was collected in a container located 2.5 feet below the rotator, washed with trichlorethylene and then acetone, and finally dried in an oven. Each run lasted approximately the same length of time.

After each run, it was noted that there was a cloud of fine powder in the room and that some metal had built up on the inside of the rotator (approx. the same for each run). However, only the material collected in the container was screen-analyzed. In each case, the collected material included a small amount of relatively coarse splat which was observed to largely result from drippings from the pour cup after each run. This larger material was separated from the line metal using a 35 mesh screen and constituted about 5 percent by weight of the total material collected except for the run at 2500 rpm where it constituted about 20 percent of the total. Screen analyses for each of these runs are set forth in Table l.

TABLE 1 PARTICLE SIZE DISTRIBUTION FINER THAN (U.S. SCREEN SIZE) RPM 35 45 80 140 230 325 400 2500 99.9 88.1 37.7 l4.5 8.7 4300 99.8 93.5 47.4 25.0 15.3 8200 100 95.5 6l.7 38.9 24.2 9800 100 97.5 87.5 7L2 49.2 33.0 25.3 10,000 99.8 93.8 75.2 54.3 34.3 2L4 19.0

The time required for the liquid metal to pass through the impingement zone was about 0.01 second. This meant that a rotator rotational velocity of 5000 rpm was required to cause the gas jet to circle theliquid metal stream once during its passage through the impingement zone. Thus, it will be understood that the rotator speeds of this Example circled the metal stream approximately 0.5, 1, 1.5, and 2 times before the latter moved out of the impingement zone. As will be seen from Table l, the fineness of the powder collected increased with increasing rotator speed and was approximately proportional to the rotator speed for powder finer than 325 mesh.

EXAMPLE 2 The procedure of Example I was repeated except that the rotator speed was set at 10,000 rpm, and the pour cup was elevated above the rotator so that the liquid tin fell about 12 inches before contacting the argon. The test results are also shown in Table 1.

In this run, the tin was in the gas impingement zone for only 0.008 second, which means that at 10,000 rpm, the gas jet circled theliquid stream about 1.3 times before the latter left the impingement zone. The rotator velocity of 10,000 rpm in this run thus corresponded to a rotator velocity of approximately 6000 rpm for the runs of Example 1 in so far as the number of revolutions of the liquid metal stream by the gas jet is concerned.

EXAMPLE 3 This Example illustrates the improvement obtained by rotating the gas jet about the liquid metal stream as compared to contacting the latter with a stationary gas jet.

Two runs were made following the procedure of Example 1 except that the metal was copper and the rotator aperture was angled at 15 to the flow axis of the liquid metal stream. The copper was heated to a temperature of about 1 150C before the runs were started. In one run (A), the liquid copper fall height before contact with the argon was about 9 in. and in the other run (B) was about 12 in. The rotator velocities were set at 9800 rpm (A) and 10,000 rpm (B). I

Two runs with stationary gas (argon) jets were also made at the same (15) impingement angle using a vee curtain nozzle which produced a gas jet wider than the liquid metal stream. The gas pressures were set at 200 psi (run C) and 400 psi (run D).

The solid collected was analyzed in the same way as described in Example 1 and the data are tabulated in Table 2.

TABLE 2 U.S. SCREEN SIZE RUN RPM 35 45 140 230 325 400 A 9800 91.6 59.0 33.4 15.8 9.9 7.0 B 10,000 99.9 86.5 56.5 34.0 15.9 10.0 7.8 C 0 100 95.2 42.9 7.6 3.5 D 0 96.9 59.9 29.2 12.4 5.7

As is apparent from Table 2, runs A and B with a rotating jet and 200 psi gas produced substantially finer powder to that obtained in run C with a stationary jet at the same gas pressure. Even an increase to 400 psi with the stationary jet (run D) failed to produce as good results as were obtained from runs A and B. Furthermore, rough estimates of the amount of gas used indicated that to produce one pound of copper powder (passing a 35 mesh screen) required approximately four times as much gas using the stationary jet at 400 psi as it did to produce a pound of copper with a similar size distribution using a rotating jet at 200 psi.

EXAMPLE 4 This Example shows that the use of a pair of rotating gas jets striking the liquid metal stream from l80-opposed positions also produces substantially improved results.

A graphite rotator having a 1.25 in. 1D. and 2.0 in. CD. was provided with a pair of diametrically-positioned, 0.125 in. holes angles at 15 to the liquid metal stream flow axis. Otherwise, the procedure and apparatus were substantially the same as used in Example 1. Runs were made at 9500, 12,000 and 15,500 rpm with liquid tin. For comparison purposes, a run was made at 9800 rpm using two holes on the same side as described in Example 1 except that they were angled at 15. The results are shown in Table 3.

TABLE 3 U.S. Screen Size RPM As will be noted from Table 3, the use of a pair of oppositely-directed jets produced substantially the same results as a single jet (2 holes) at substantially the same rotational velocities of 9500 and 9800 rpm, respectively. However, very little improvement was obtained with the oppositely-directed jets as the rotator velocity was increased.

EXAMPLE This Example shows the effect of offsetting the gas jet and the flow axis of the liquid metal stream from each other.

A run was carried out substantially as described in Example 1 except that the liquid stream was offset from the axis of rotation of the rotator by about 0.5 in. The results are shown in Table 4.

TABLE 4 US. Screen Size RPM As will be noted from a comparison of the results of Table 4 with the run at 9800 rpm of Example 1, there is a substantial decrease in the amount of the finer powder when the gas jet and liquid metal stream are offset from each other.

I claim: 1. A method for producing powder from a liquid metal stream comprising the steps of:

providing at least one jet of gas capable of penetrating the surface of said liquid metal stream;

rotating each said jet about said liquid metal stream at an angle between about 5 and about with respect to the flow direction of said stream to strike said liquid metal stream in an impingement zone at a rotational velocity sufficient to circle said liquid metal stream at least once before the liquid metal leaves said impingement zone to cause said liquid metal to break up into droplets;

collecting said droplets as a powder after said droplets have cooled and solidified.

2. The method of claim 1 wherein each said jet has a width less than that of said li uid metal stream.

3. The method of claim wherein the axis of each said jet is coplanar with the flow axis of said liquid metal stream so that each said jet is centered on said liquid metal stream as each said jet rotates.

4. The method of claim 1 wherein a single jet of gas is employed.

5. The method of claim 4 wherein said jet is oriented at an angle between about 5 and about 45 with respect to said flow direction of said liquid metal stream.

6. The method of claim 1 wherein a pair of diametrically-opposed jets are employed.

7. The method of claim 1 wherein said gas is inert to said liquid metal.

a: a: :0: w: 1' 

2. The method of claim 1 wherein each said jet has a width less than that of said liquid metal stream.
 3. The method of claim 2 wherein the axis of each said jet is coplanar with the flow axis of said liquid metal stream so that each said jet is centered on said liquid metal stream as each said jet rotates.
 4. The method of claim 1 wherein a single jet of gas is employed.
 5. The method of claim 4 wherein said jet is oriented at an angle between about 5* and about 45* with respect to said flow direction of said liquid metal stream.
 6. The method of claim 1 wherein a pair of diametrically-opposed jets are employed.
 7. The method of claim 1 wherein said gas is inert to said liquid metal. 